1. Field of the Invention
This invention relates to storage batteries and more particularly to, a material useful as a separator of the batteries. The batteries using the material as a separator exhibit a long life and can suppress self discharge to a minimum even when placed under high temperature conditions. The invention also relates to a method for making the separator material.
2. Description of the Prior Art
In recent years, various portable electronic devices or equipments have been drastically developed with an increasing necessity for memory backup for these devices. This brings about a great demand for batteries to be used as a power source for the devices. However, as the use of the batteries has been extended, there is a great possibility of the batteries being employed under severer use conditions, such as high temperature conditions, than those experienced ever before. Accordingly, there is a demand for improvements of the batteries or cells particularly with respect to life characteristics including a self discharge characteristic. In electronic devices, nickel-cadmium storage batteries have been frequently employed. The nickel-cadmium storage battery is known as a battery which undesirably involves a large self discharge. The improvement of the self discharge characteristic of the battery is one of problems to solve. For instance, when an ordinary nickel-cadmium battery has been allowed to stand at 45.degree. C. after having been fully charged, the discharge capacity reduces to less than 1/3 of an initial capacity, leaving a problem in use immediately after the standing.
To solve the problem, studies on the self discharge characteristic particularly of nickel-cadmium storage batteries have been extensively made. The results of recent studies reveal that one of important factors of reducing the self discharge concerns with characteristics of a separator used in the battery. More particularly, it has been found that the increase of the self discharge owes greatly to the presence of impurities caused by decomposition of a polyamide fiber separator ordinarily used for this purpose.
Attention has now been paid to olefin resin fibers, such as polypropylene fibers, as a material for the separator. This is because the olefin resin fibers have a good heat resistance and are so stable that they do not decompose in highly concentrated alkaline solutions even at high temperatures. However, the olefin resin fibers are disadvantageous in that they are poor in hydrophilicity with an attendant problem that the liquid retention necessary for the separator is not satisfactory. For solving this problem, there have been proposed the following improvements of the polyolefin resin fibers where the fibers are imparted with hydrophilicity.
(1) Deposition of surface active agents on the fiber surfaces.
(2) Introduction of hydrophilic groups such as --COOH groups or --CHO groups to the resin fibers by irradiation of plasma.
(3) Graft polymerization of polyolefin resins with hydrophilic monomers, such as, for example, acrylic acid.
(4) Introduction of sulfonic groups (-SO.sub.3 H) into the resin fibers by immersion of the resin fibers into fuming sulfuric acid or concentrated sulfuric acid under proper temperature and concentration conditions.
However, these techniques involve the following problems or disadvantages.
With (1), since a separator placed in a highly concentrated alkaline aqueous solution at high temperatures contacts with oxygen gas generated at the positive electrode when the battery is overcharged, the surface active agent applied to the resin fibers separates from the fibers. The separated surface active agent dissolves in the alkaline aqueous solution where it serves as an impurity. This eventually facilitates the self discharge.
This is true of the cases of (2) and (3) where severe oxidation within the battery causes carboxylic acid ions and acrylic acid ions to dissolve in the alkaline aqueous solution, respectively. These ions increase the self discharge of the battery.
In contrast, with (4), little impurities or ions dissolve in the aqueous alkaline solution as in (1) to (3), so that the introduction of sulfate groups in very effective in suppressing the self discharge. However, when fibers made of one kind of polyolefin resin are used or polyolefin resin fibers whose inside is susceptible to oxidation are used, uniform sulfonation has to be effected within a very narrow range of treating conditions including a temperature and a concentration of fuming or concentrated sulfuric acid, and an immersion time. The sulfonation under these severe conditions is very difficult to carry out in an industrial sense, thus being not advantageous from the standpoint of industrial feasibility. Moreover, although it is important that the resin fibers have hydrophilic groups substantially uniformly distributed throughout the fibers for use as a separator, polyolefin resins have generally unsaturated hydrocarbon moieties susceptible to sulfonation, which are distributed only non-uniformly. If the fibers are sulfonated as a whole, the fibers will be sulfonated or oxidized to the inside thereof. In an extreme case, the inside portions may suffer carbonization. This results in a lowering of mechanical strength of the separator itself. Especially, when bonding portions of the fibers are carbonized, the lowering of the mechanical strength becomes considerable, thus presenting a serious problem.